1. Technical Field
This invention relates to the field of load sensors and More particularly to an improved high-payload six-axis load sensor.
2. Description of the Related Art
In automatically controlled systems, and particularly in robotic machinery, it is desirable to monitor the forces and moments being generated at the work site between the motive, or drive power element, and the driven element of the operating unit, frequently referred to as an xe2x80x9carmxe2x80x9d. Force and moment measurements, and feedback for control in response to such measurements, have heretofore been accomplished by a number of methods. For example, individual axial forces have been measured with a standard moment measuring load transducer.
Load transducers are electromechanical transducers that translate force or weight into voltage. This change in voltage produces, in the read-out instrumentation, a repeatable deflection or indication that can be calibrated directly in terms of the load applied to the load transducer. Construction of the load transducer utilizes all the advantages of bonded foil strain gauges. Sealed within the load transducer are sets of matched strain gauges bonded to a high strength element, machined to close tolerances. The strain gauges are electrically connected to form a balanced Wheatstone bridge. Furthermore, additional compensation resistors are added to the circuit for maintaining the accuracy of the Wheatstone bridge over a wide temperature range. The principle of operation depends upon the deflection of the strain gauge filament, creating a change in its resistance, thereby unbalancing the Wheatstone bridge circuit. As a result, for a given input voltage, the output voltage of the bridge varies proportionally with the load. Consequently, the change can be read on appropriate instrumentation.
For simultaneous measurement of more than one load or moment, single force or single moment load transducers have been assembled with suitable mechanical devices to achieve the required loading measurements. Specifically, one multi-component element can be constructed to measure: multiple loads or moments applied to the multi-component element. When larger sizes or ranges are necessary, a force platform, or table, can be constructed, the table having attached thereto more than one multi-component element. Presently, many such prefabricated systems are commercially available.
For instance, U.S. Pat. No. 3,640,130 issued to Spescha et al. on Feb. 8, 1972 for FORCE AND MOMENT ARRANGEMENTS discloses a measuring unit embodying a plurality of force-measuring load transducers. The Spescha et al. apparatus permits the separation of complicated force and moment effects upon a body to be investigated and maps the same to desired coordinate axes and planes. Spescha et al. note that for the general case, six measuring quantities are required to express a force vector engaging a test point in any spatial relationship. Thus, the six measuring quantities must be measured simultaneously.
In consequence, a condition for reliable performance of such an apparatus is a highly accurate assembly of very rigid measuring load transducers in accordance with the requirements of the desired ordinate directions. Recognizing the need for very rigid measuring transducers, the Spescha et al. device particularly requires each measuring load transducer to be rigidly secured between two plates. However, the rigid construction of the apparatus precludes the low cost maintenance and repair of each measuring load transducer rigidly secured between the two plates. In particular, a poorly performing measuring load transducer cannot easily be replaced with a proper measuring load transducer.
Similar problems exist with other six-axis force-torque sensors. For instance, U.S. Pat. No. 3,956,930 issued to Shoberg on May 18, 1976 for DRIVELINE TORQUE AND/OR THRUST SENSOR discloses a sensor transducer for measuring the torque and thrust forces in a driveline system having a U-type coupling joint. The Shoberg apparatus includes a force transducer having a substantially circular body. Significantly, the thrust gauges are stacked or laminated in pairs on flexure struts incorporated as part of the transducer body. Moreover, torque measuring gauges are laminated on opposite surfaces of the struts. Thus, like the Spescha et al. device, the gauges are permanently incorporated as part of the sensor and cannot easily be removed for repair.
Likewise, U.S. Pat. No. 4,488,441 issued to Ramming et al. on Dec. 18, 1984 for APPARATUS FOR SIMULTANEOUS MEASUREMENT OF MUTUALLY PERPENDICULAR FORCES AND MOMENTS discloses a device suitable for controlling a robot operating arm having a drive linked between a driving member and a driven member which can determine forces and moments acting to resist movement therebetween. The Ramming et al. sensor includes a pair of generally parallel plate or base members which are joined by curved arch segments circumferentially spaced around the periphera of the plates and are normal thereto. As in Shoberg, however, strain gauges are permanently affixed to one or more surfaces of the arch segments. Thus, like the Spescha et al. device, the strain gauges cannot easily be removed for repair.
Finally, U.S. Pat. No. 4,094,192 issued to Watson et al. on Jun. 13, 1978 for METHOD AND APPARATUS FOR SIX DEGREE OF FREEDOM FORCE SENSING discloses a system for processing outputs from strain gauges attached to a structure to derive the desired force and torque components in an orthogonal three-axis coordinate system. Like the Ramming et al. sensor, the device described in Watson et al. includes strain gauges permanently affixed to intermediate sections connecting two force sensing sections. As a result, the strain gauges easily cannot be removed and repaired. Thus, a need exists for a high payload, low cost, repairable six-axis force-torque sensor.
An improved high-payload six-axis load sensor can comprise: a table for receiving an applied load; a base; and, at least three shear-pin load transducers removeably mounted between the table and the base. Notably, the three shear-pin load transducers can measure reaction forces between the table and the base, the reaction force having been produced by the applied load. Preferably, each shear-pin load transducer is responsive to shear forces imparted along two axes perpendicular to an axis of minimum sensitivity characteristic of the transducer. Responsive to an applied shear force, each shear-pin load transducer can produce an electrical signal proportional to the reaction forces. Thus, the improved high-payload six-axis load sensor can further comprise computing means for receiving the proportional electrical signals and computing the applied load corresponding to the proportional electrical signals. The computing means further can express the computed load in terms of a three-dimensional XYZ Cartesian coordinate system.
Preferably, the shear-pin load transducers are symmetrically mounted in mounting holes about the table. In addition, the table can be a low-profile disc. As such, each of the shear-pin load transducers can be radially positioned apart from each other shear-pin load transducer about a circumference of the disc. In addition, the table can define at least two flexures, each flexure corresponding to a shear-pin load transducer removably mounted between the table and the base. Each flexure can be formed by removing material from the table to form one or more thin beam structures. The number of flexures formed in the table depends on the maximum load anticipated, material employed, and desired stiffness. The present invention incorporates four flexures for each shear-pin load transducer.
In forming each flexure, the flexure preferably can be positioned perpendicular to an axis of minimum sensitivity characteristic of the removeably disposed shear-pin load transducer. In consequence, the flexure can minimize unmeasured axial loading on the shear-pin load transducer. Still, one skilled in the art will recognize that means other than flexures can be employed to prevent axial loading of the shear-pin load transducer. For example, it may be practical to place a low friction sliding coupling along the axis of the shear-pin load transducer.
Each shear-pin load transducer can further include a spherical bearing embedded in the table, the spherical bearing defining a cylindrical passage. In accordance with its design, each spherical bearing can enclose a length of the shear-pin load transducer inserted through the cylindrical passage of the spherical bearing when removeably mounted in the table. Without the spherical bearing, small deflections and distortions in the table caused by a load can cause slight angular misalignment of the mounting holes. The misalignment can cause unequal loading of the shear-pin load transducer along its length which can in turn degrade the accuracy of a resulting measurement. Thus, the spherical bearing can reduce moment loads applied to the shear-pin load transducer, and therefore improve measurement accuracy.
A method for computing an applied load in a high-payload six-axis load sensor having three shear-pin force transducers comprises the steps of first measuring a non-linear output voltage from each strain gauge in each of the three shear-pin force transducers contained in the high-payload six-axis load sensor. Second, the inventive method can transform each non-linear output voltage to a non-linear gauge voltage. Third, for each of the three shear-pin force transducers, the method can linearize the measured non-linear voltages, the linearization forming a linear vertical force component and a linear horizontal force component for each of the three shear-pin force transducers. Fourth, for each vertical force component and horizontal force component formed from a measured non-linear voltage, the method can remove pin mounting bias from the vertical force component and the horizontal force component, the removing step forming a modified vertical force component and a modified horizontal force component. Finally, the method can compute an applied load from the modified vertical force components and modified pin horizontal force components.
An improved high-payload six-axis load sensor in accordance with the inventive arrangement satisfies the long-felt need of the prior art by providing a simple, low cost, field repairable six-axis load sensor which is capable of very high payloads. Specifically, the invention can enable heavy-load robotic manipulators that are scalable upward from a 5,000 lbs. force and 5,000 ft-lbs. torque payload capacity to over a 100,000 lbs. force and 100,000 ft-lbs torque capacity. Thus, the inventive arrangements provide a novel design for a six-axis load sensor. The inventive arrangements have advantages over all six-axis load sensors, and provides a low cost, repairable, high capacity, force and torque sensor for measuring the forces and moments applied to a manipulator, machine tool or fixture.